The applicable formula is
A = P(r/12)/(1 -(1+r/12)^(-12n))
where P is the principal amount,
r is the annual interest rate (compounded monthly), and
n is the number of years.
Using the formula, we find
A = 84,400*(0.04884/12)/(1 -(1+0.04884/12)^(-12*15))
= 84,400*0.00407/(1 -1.00407^-180)
= 343.508/0.518627
≈ 662.34
The monthly payment on a mortgage of $84,400 for 15 years at 4.884% will be
$662.34